Exhaust gas purification apparatus of an internal combustion engine

ABSTRACT

In an exhaust gas purification apparatus of an internal combustion engine, there is provided a technique that is able to quickly raise the temperature of a catalyst arranged at a downstream side by raising the temperature of a catalyst arranged at an upstream side in a quick manner. The apparatus is provided with an exhaust gas purification catalyst, a plurality of catalysts that are arranged at an upstream side of the exhaust gas purification catalyst and have oxidizing ability, a fuel supply device that supplies fuel to one catalyst which is arranged at the most upstream side, and a heating device that heats the one catalyst, wherein the plurality of catalysts having oxidizing ability are arranged in an exhaust passage in series to the direction of flow of an exhaust gas, and the more upstream side the catalysts are arranged at, the smaller the cross-sectional areas of the catalysts formed by cutting planes which are orthogonal to a central axis of the exhaust passage are made.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification apparatusof an internal combustion engine.

BACKGROUND ART

There has been known a technique in which heat can be generated bysupplying a reducing agent to an oxidation catalyst, so that thetemperature of an exhaust gas can thereby be raised, as a result ofwhich the temperature of a catalyst arranged at a location downstream ofthe oxidation catalyst is raised (for example, see a first patentdocument).

However, at the time of cold start of an internal combustion engine, thetemperature of the oxidation catalyst is low, so the reducing agenthardly reacts with the oxidation catalyst. Therefore, heating theoxidation catalyst by means of a heater, etc., is carried out. However,because the heat generated by the heater, etc., is taken by the exhaustgas as the exhaust gas passes through the oxidation catalyst, thetemperature rise of the oxidation catalyst becomes slow. On the otherhand, when an amount of heat more than the heat taken by the exhaust gasis to be generated, it has been necessary to enlarge the size of theoxidation catalyst, or to increase the amount of electric power used bythe heater, etc.

PRIOR ART REFERENCES Patent Documents

-   First Patent Document: Japanese patent application laid-open No.    2005-127257-   Second Patent Document: Japanese patent application laid-open No.    2004-162611-   Third Patent Document: Japanese patent application laid-open No.    H6-106068-   Fourth Patent Document: Japanese patent application laid-open No.    2003-120264-   Fifth Patent Document: Japanese patent application laid-open No.    2006-161629-   Sixth Patent Document: Japanese patent application laid-open No.    H9-504349

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made in view of the above-mentionedproblems, and has for its object to provide a technique which is capableof quickly raising the temperature of a catalyst arranged at adownstream side by raising the temperature of a catalyst arranged at anupstream side in a quick manner.

Means for Solving the Problems

In order to achieve the above-mentioned object, an exhaust gaspurification apparatus of an internal combustion engine according to thepresent invention adopts the following measures.

That is, the exhaust gas purification apparatus of an internalcombustion engine according to the present invention is characterized bycomprising:

an exhaust gas purification catalyst that is arranged in an exhaustpassage of the internal combustion engine for purifying an exhaust gas;

a plurality of catalysts that are arranged at an upstream side of saidexhaust gas purification catalyst and have oxidizing ability;

a fuel supply device that supplies fuel to one of said plurality ofcatalysts having oxidizing ability which is arranged at the mostupstream side thereof;

a heating device that heats said one catalyst;

wherein

said plurality of catalysts having oxidizing ability are arranged in theexhaust passage in series to the direction of flow of the exhaust gas,and the more upstream side the catalysts are arranged at, the smallerthe cross-sectional areas of the catalysts formed by cutting planeswhich are orthogonal to a central axis of the exhaust passage are made.

Here, the more downstream side the plurality of catalysts havingoxidizing ability are arranged at the larger the cross-sectional areasof the catalysts become, so a gas passing through the inside of acatalyst which is arranged at an upstream side, and a gas passingthrough the outside of the catalyst which is arranged at the upstreamside, flow into a catalyst which is arranged at a downstream sidethereof. The gas having passed through the inside of the catalyst whichis arranged at the upstream side becomes high in temperature due to thereaction of fuel in this catalyst. In addition, a part of fuel which hasnot yet reacted is included in this gas. Moreover, oxygen is consumedinside the catalyst which is arranged at the upstream side, so theoxygen concentration of the gas flowing out of this catalyst becomeslow. As a result, in the gas having passed through the inside of theupstream side catalyst, the fuel, which is contained therein and whichcan be made to react in the downstream side catalyst, decreases. On theother hand, a large amount of oxygen is contained in the gas havingpassed through the outside of the catalyst which is arranged at theupstream side. Thus, by taking this gas into the downstream sidecatalyst, oxidation of fuel can be facilitated in this downstream sidecatalyst.

Here, the one catalyst has a small volume, so the temperature thereof isquickly raised by means of the heating device. Then, when thetemperature of the one catalyst is raised by means of the heatingdevice, fuel can be made to react in this one catalyst. As a result ofthis, the temperature of the exhaust gas flowing out of the one catalystrises, so the temperature of the following catalyst, which is arrangedat the downstream side of this one catalyst, also rises. In other words,in the downstream side catalyst, the temperature thereof is caused torise abruptly by means of the heat given from the upstream side catalystand the heat generated in the downstream side catalyst. Thus, by raisingthe temperatures of the plurality of catalysts in a sequential manner,downstream side catalysts can be raised to high temperatures. Then, alarge amount of heat can be generated in the plurality of catalystshaving oxidizing ability, so that the temperature of the exhaust gaspurification catalyst can finally be raised.

In the present invention, said exhaust gas purification catalyst can becomposed of including a selective reduction type NOx catalyst which usesurea or ammonia as a reducing agent; and

an injection device can be provided which injects said reducing agenttoward the exhaust gas flowing out of an other catalyst which isarranged at the most downstream side among said plurality of catalystshaving oxidizing ability.

In the present invention, the temperature of the other catalyst which isarranged at the most downstream side can be raised quickly, and theother catalyst becomes a high temperature. As a result of this, thetemperature of the exhaust gas flowing out of the other catalyst alsobecomes a high temperature, so by injecting the reducing agent towardthe exhaust gas flowing out of the other catalyst, the evaporation ofthe reducing agent can be facilitated. In addition, the reducing agentcan be dispersed to a wide range in the exhaust gas. Here, note that,the other catalyst may have its cross-sectional area smaller than thatof the exhaust passage.

In the present invention, a unit is provided which measures or estimatesthe temperature of the exhaust gas flowing out of said other catalyst,wherein when the temperature of the exhaust gas flowing out of saidother catalyst is equal to or higher than a predetermined value, thereducing agent is caused to be injected from said injection device.

The predetermined value can be a temperature at which the reducing agentcan be evaporated, or a temperature at which the reducing agent can bedispersed in an effective manner. In other words, even if the reducingagent is injected at the time when the temperature of the exhaust gasflowing out of the other catalyst is low, evaporation or dispersion ofthe reducing agent will not be able to be expected, but there will alsobe a fear that the reducing agent may adhere to the exhaust gaspurification catalyst. Accordingly, in cases where the temperature ofthe exhaust gas flowing out of the other catalyst is equal to or higherthan the predetermined value, the reducing agent is caused to beinjected.

In the present invention, before starting of said internal combustionengine, fuel can be supplied to said one catalyst from said fuel supplydevice, and said one catalyst can be heated by means of said heatingdevice.

In other words, the temperature of the one catalyst is caused to risebefore the internal combustion engine starts. On the contrary, theinternal combustion engine may be started after the temperature of theone catalyst is caused to rise up to a prescribed temperature. Withthis, at the time of the next starting of the internal combustionengine, too, it is possible to raise the temperature of a catalyst(s)downstream of said one catalyst in a quick manner. As a result of this,in the exhaust gas purification catalyst, purification of the exhaustgas can be made at an early stage.

In the present invention, after the starting of the internal combustionengine is commenced, the amount of fuel supplied from said fuel supplydevice can be caused to increase according to the time elapsed.

In other words, the amount of fuel capable of being oxidized in theplurality of catalysts having oxidizing ability increases in accordancewith the rising temperature of the one catalyst and/or the risingtemperatures of the catalysts having oxidizing ability which arearranged at the downstream side thereof. If the amount of fuel to besupplied is increased according to this, the amount of heat generated inthe plurality of catalysts having oxidizing ability will be able to beincreased, so that the temperature of the exhaust gas purificationcatalyst can be raised in a quick manner. In addition, the supply of thereducing agent to the exhaust gas purification catalyst can be made atan early stage.

In the present invention, a unit is provided which measures or estimatesthe temperature of said exhaust gas purification catalyst, wherein whenthe temperature of said exhaust gas purification catalyst rises to theprescribed temperature, the supply of fuel from said fuel supply deviceto said one catalyst can be stopped, and the heating of said onecatalyst by said heating device can be stopped.

If the reducing agent can be made to react in the exhaust gaspurification catalyst, it will become unnecessary to raise thetemperature of the catalysts having oxidizing ability. If the supply offuel or the heating by the heating device is stopped, fuel economy orefficiency can be improved. In addition, overheating of the catalystscan be suppressed.

In the present invention, when the amount of the exhaust gas becomesmore than a prescribed amount during the time fuel is being suppliedfrom said fuel supply device, the amount of fuel supplied from said fuelsupply device can be restricted.

This prescribed amount can be an amount at which there is a fear thatthe fuel supplied by the fuel supply device may adhere to the exhaustgas purification catalyst. In other words, when the amount of theexhaust gas increases, the time for fuel to pass through the pluralityof catalysts having oxidizing ability becomes shorter, so the fuelbecomes more difficult to be oxidized in the catalysts. In other words,the amount of fuel which passes through the plurality of catalystshaving oxidizing ability without being oxidized therein increases. Thus,if the fuel which has passed through the catalysts having oxidizingability adheres to the exhaust gas purification catalyst, there will bea fear that the purification ability of the exhaust gas purificationcatalyst may be decreased. On the other hand, by restricting the amountof fuel to be supplied, it is possible to suppress the fuel fromadhering to the exhaust gas purification catalyst. Here, note that theamount of fuel to be supplied may be decreased according to the amountof the exhaust gas. In this case, the amount of fuel to be supplied maybe decreased in a continuous manner or in a stepwise manner according tothe amount of the exhaust gas. In addition, when the amount of theexhaust gas becomes equal to or more than the prescribed amount, thesupply of fuel from the fuel supply device may be stopped.

Effect of the Invention

According to an exhaust gas purification apparatus of an internalcombustion engine related to the present invention, it is possible toquickly raise the temperature of a catalyst arranged at a downstreamside by raising the temperature of a catalyst arranged at an upstreamside in a quick manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing the schematic construction of an internalcombustion engine and its exhaust system to which an exhaust gaspurification apparatus of an internal combustion engine according to afirst embodiment of the present invention is applied.

FIG. 2 is a construction view of a temperature raising device.

FIG. 3 is a flow chart showing a flow for temperature raising control ona NOx catalyst at the time of starting of the engine according to anembodiment of the present invention.

FIG. 4 is a flow chart showing a flow for temperature raising control onthe NOx catalyst after the engine has been started according to theembodiment.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, reference will be made to a specific embodiment of anexhaust gas purification apparatus of an internal combustion engineaccording to the present invention based on the attached drawings.

First Embodiment

FIG. 1 is a view showing the schematic construction of an internalcombustion engine and its exhaust system to which an exhaust gaspurification apparatus of an internal combustion engine according tothis embodiment of the present invention is applied. An internalcombustion engine 1 shown in FIG. 1 is a four-cycle diesel engine of awater cooled type.

In addition, an exhaust passage 2 is connected to the internalcombustion engine 1. In the middle of the exhaust passage 2, atemperature raising device 3 and a selective reduction type NOx catalyst4 (hereinafter referred to as a NOx catalyst 4) are sequentiallyarranged in this order from an upstream side. The NOx catalyst 4 reducesNOx in an exhaust gas in a selective manner by supplying urea or ammoniaas a reducing agent. Here, note that in this embodiment, the NOxcatalyst 4 corresponds to an exhaust gas purification catalyst in thepresent invention.

FIG. 2 is a construction view of the temperature raising device 3. Thetemperature raising device 3 is provided with four oxidation catalystsincluding a first catalyst 31, a second catalyst 32, a third catalyst33, and a fourth catalyst 34, which are arranged in a sequential mannerfrom an upstream side to a downstream side, with appropriate distancesbetween adjacent catalysts, respectively. Here, note that at least twooxidation catalysts should be provided. In addition, these catalystsshould just have oxidizing ability, and may be three-way catalysts orocclusion reduction type NOx catalysts. These four oxidation catalystsare of cylindrical shapes, respectively, and the central axis of eachcatalyst is located on the central axis of the exhaust passage 2.Moreover, the more upstream side the catalysts are arranged at, thesmaller do the cross-sectional areas of the catalysts when thesecatalysts are cut by planes which are orthogonal to the central axis ofthe exhaust passage 2 becomes. In other words, the cross-sectional areaof the first catalyst 31 is the smallest, and the cross-sectional areaof the fourth catalyst 34 is the largest. The cross-sectional area ofthe fourth catalyst 34 is smaller than the passage sectional area of theexhaust passage 2. Also, the more downstream side the oxidationcatalysts are arranged at, the larger the volumes of the oxidationcatalysts are. In addition, the second catalyst 32, the third catalyst33, and the fourth catalyst 34 are formed with the cylindrical guides321, 331, 341, respectively, which extend toward the upstream side fromthe outer peripheries of the individual catalysts, respectively. Each ofthese guides 321, 331, 341 extends up to a more upstream side than thedownstream end of the catalyst which is arranged at an immediatelyupstream side of each of the catalysts. Here, note that in thisembodiment, the first catalyst 31, the second catalyst 32, the thirdcatalyst 33, and the fourth catalyst 34 correspond to a plurality ofcatalysts having oxidizing ability in the present invention.

A first injection valve 35 for injecting fuel is arranged at theupstream of the first catalyst 31. The first injection valve 35 has itsnozzle hole directed to the center of an upstream end face of the firstcatalyst 31. In addition, the first catalyst 31 is provided with aheater 36 which serves to heat the first catalyst 31. This heater 36generates heat by being supplied with electric power. In other words,the first injection valve 35 is arranged at a more upstream side thanthe most upstream side oxidation catalyst, and the heater 36 is mountedon the most upstream side oxidation catalyst. The nozzle hole of thefirst injection valve 35 may be arranged to be directed to a place whichis to be heated by the heater 36. Here, note that in this embodiment,the first catalyst 31 corresponds to one catalyst in the presentinvention. Also, in this embodiment, the first injection valve 35corresponds to a fuel supply device in the present invention. Further,in this embodiment, the heater 36 corresponds to a heating device in thepresent invention.

A second injection valve 37 for injecting a liquid in which urea orammonia is contained is arranged in the exhaust passage 2 at a locationin the vicinity of the fourth catalyst 34. The liquid having urea orammonia contained therein acts as a reducing agent in the NOx catalyst4. The second injection valve 37 has its nozzle hole directed to astream of exhaust gas which flows out of the fourth catalyst 34. Here,note that in this embodiment, the fourth catalyst 34 corresponds to another catalyst in the present invention. Also, in this embodiment, thesecond injection valve 37 corresponds to an injection device in thepresent invention.

In addition, a first temperature sensor 38 for measuring the temperatureof the exhaust gas is arranged at a location downstream of the fourthcatalyst 34. By this first temperature sensor 38, the temperature of thefourth catalyst 34 or the temperature of the exhaust gas flowing out ofthe fourth catalyst 34 is measured. Here, note that by the firsttemperature sensor 38, the temperature of the temperature raising device3 or the temperature of the exhaust gas flowing into the NOx catalyst 4can also be measured. In addition, a second temperature sensor 13 formeasuring the temperature of the exhaust gas is arranged in the exhaustpassage 2 at a location downstream of the fourth catalyst 4. Thetemperature of the NOx catalyst 4 can also be measured by this secondtemperature sensor 13. Here, note that in this embodiment, the firsttemperature sensor 38 corresponds to a unit which measures or estimatesthe temperature of the other catalyst in the present invention.Moreover, in this embodiment, the second temperature sensor 13corresponds to a unit which measures or estimates the temperature of theexhaust gas purification catalyst in the present invention.

Further, a crank angle sensor 11 for measuring the number of revolutionsper unit time of the internal combustion engine 1 is mounted on theinternal combustion engine 1.

In the internal combustion engine 1 constructed as stated above, thereis arranged in combination therewith an ECU 5 which is an electroniccontrol unit for controlling the internal combustion engine 1. This ECU5 is a unit that controls the operating state of the internal combustionengine 1 in accordance with the operating conditions of the internalcombustion engine 1 and/or driver's requirements.

Besides the above-mentioned sensors, an accelerator opening sensor 15,which is able to detect an engine load by outputting an electricalsignal corresponding to an amount by which a driver depressed anaccelerator pedal 14, and a switch 12, which serves to start theinternal combustion engine 1, are connected to the ECU 5 through wiring,and the output signals of the variety of kinds of sensors are inputtedto the ECU 5. With the switch 12 being operated by the driver, the ECU 5starts up the internal combustion engine 1.

On the other hand, the first injection valve 35 and the second injectionvalve 37 are connected to the ECU 5 through electrical wiring, so thatthese valves are controlled by means of the ECU 5.

With the arrangement of the four oxidation catalysts 31, 32, 33, 34 asin this embodiment, most of the exhaust gas having passed through anupstream side oxidation catalyst flows into the following oxidationcatalyst at the downstream side thereof. In other words, the gas havingflown out of each of the oxidation catalysts flows through the inside ofthe guide formed in each of the following downstream side oxidationcatalysts, and flows into the downstream side oxidation catalysts. Onthe other hand, because there is a gap between each of the upstream sideoxidation catalysts and the guide of the following each downstream sideoxidation catalyst, a part of the exhaust gas having passed through theoutside of each upstream side oxidation catalyst flows into thefollowing each downstream side oxidation catalyst.

Here, when the heater 36 is energized with electric power and fuel isinjected from the first injection valve 35, the fuel reacts in the firstcatalyst 31 to generate heat. As a result of this, the temperature ofexhaust gas is raised. Then, when the exhaust gas thus heated flows intothe second catalyst 32, the temperature of the second catalyst 32 israised. In the exhaust gas flowing into this second catalyst 32, thereis contained fuel which did not react or reacted insufficiently in thefirst catalyst 31. However, when the exhaust gas passes through theinterior of the first catalyst 31, oxygen reacts with the fuel in thefirst catalyst 31, so there remains only a small amount of oxygen in theexhaust gas which flows out of the first catalyst. On the other hand, apart of the exhaust gas, which passed through the outside of the firstcatalyst 31, also flows into the second catalyst 32. A lot of oxygen iscontained in the exhaust gas which passed through the outside of thisfirst catalyst 31. In other words, the fuel flowing out of the firstcatalyst 31 and the exhaust gas having much oxygen contained thereinbecause of passing through the outside of the first catalyst 31 flowinto the second catalyst 32. Therefore, in the second catalyst 32, too,fuel and oxygen react with each other to generate heat. As a result, thetemperature of the exhaust gas is further raised. Such things also occurin the third catalyst 33 and the fourth catalyst 34, too.

In other words, the temperature of the exhaust gas is raised in each ofthe oxidation catalysts due to the oxygen taken in by each oxidationcatalyst. With this, the temperature of their downstream side catalystscan be further raised. For example, with such an arrangement, thetemperature of the exhaust gas which arrives at the NOx catalyst 4 canbe made higher than that in cases where one single oxidation catalyst isprovided which has the same volume as the sum total of the volumes ofthe four oxidation catalysts and in cases where the same amount of fuelis supplied. That is, according to this embodiment, the temperature ofthe NOx catalyst 4 can be quickly raised with a smaller amount of fuel.

In addition, in this embodiment, by injecting the reducing agentinjected from the second injection valve 37 toward the exhaust gasflowing out of the fourth catalyst 34, evaporation of the reducing agentis made to facilitate or the reducing agent is made to disperse in awide area. Here, the fourth catalyst 34 is made high in temperature dueto the heat generated in the three catalysts, which are arranged at theupstream side of the fourth catalyst 34, and the heat generated in thisfourth catalyst 34. Therefore, by injecting the reducing agent towardthe exhaust gas flowing out of this fourth catalyst 34, the reducingagent can be evaporated and dispersed in a quick manner. Here, note thatthe reducing agent may be injected from the second injection valve 37only in cases where the evaporation and dispersion of the reducing agentcan be carried out to a sufficient extent. In other words, in caseswhere the temperature of the exhaust gas flowing out of the fourthcatalyst 34 is equal to or higher than a threshold value, it is decidedthat the evaporation and dispersion of the reducing agent can be done toa sufficient extent, and the reducing agent may be injected. Thisthreshold value has beforehand been obtained through experiments, etc.

Then, in this embodiment, in cases where the temperature of the NOxcatalyst 4 is lower than a lower limit value of an activationtemperature thereof at the time of cold starting of the internalcombustion engine 1, etc., the following control is carried out so as toquickly raise the temperature of the NOx catalyst 4.

FIG. 3 is a flow chart showing a flow for temperature raising control onthe NOx catalyst 4 at the time of engine starting according to thisembodiment. This routine is executed at the time of starting of theinternal combustion engine 1. Here, note that in this embodiment, evenwhen the driver operates the switch 12 in order to start the internalcombustion engine 1, the temperature of the first catalyst 31 is firstraised without immediately starting the internal combustion engine 1.

In step S101, the ECU 5 determines whether the temperature of the NOxcatalyst 4 is lower than the lower limit value (e.g., 150 degrees C.) ofthe activation temperature. In other words, it is determined whetherreduction of NOx can not be carried out in the NOx catalyst 4. Forexample, when the temperature obtained by the second temperature sensor13 is lower than a threshold value, it is assumed that the temperatureof the NOx catalyst 4 is lower than the lower limit value of theactivation temperature.

In cases where an affirmative determination is made in step S101, theroutine advances to step S102, whereas in cases where a negativedetermination is made, this routine is ended. In cases where thisroutine is ended, the internal combustion engine 1 is startedimmediately. Then, the reducing agent is injected from the second fuelinjection valve 37, without being accompanied by a temperature rise ofexhaust gas by means of the temperature raising device 3, so that NOx ispurified.

In step S102, the ECU 5 starts energization of the heater 36 and theinjection of fuel from the first injection valve 35. At this time, theinternal combustion engine 1 is not operated, so there is no flow ofexhaust gas. Therefore, it is suppressed that the heat generated in thefirst catalyst 31 is taken by the exhaust gas, and hence the temperatureof this first catalyst 31 rises quickly.

In step S103, the ECU 5 starts the internal combustion engine 1. Inother words, fuel is supplied to combustion chambers of the internalcombustion engine 1. Alternatively, when the temperature of the firstcatalyst 31 reaches a prescribed temperature by executing the processingof step S102, the internal combustion engine 1 may be started. Also,alternatively, when a prescribed period of time has elapsed after theexecution of the processing of step S101, the internal combustion engine1 may be started.

In step S104, the ECU 5 executes energization control on the heater 36,and injection control on the first injection valve 35. In this step, theheater 36 and the first injection valve 35 are controlled by the numberof engine revolutions per minute, the engine load, and the elapsed timefrom the start of execution of this step. In other words, energizationof the heater 36 is carried out, or energization of the heater 36 isstopped, or the amount of fuel injected from the first injection valve35 is adjusted. Here, the amount of heat generated and the temperaturein each catalyst change according to the number of engine revolutionsper minute, the engine load, and the elapsed time from the start of thisstep, so the amount of heat generation is adjusted according to thesefactors. In general, the longer the elapsed time, the higher the degreeof activity of each of the oxidation catalysts becomes, so the amount offuel injected from the first injection valve 35 is caused to increase.Here, the fuel injection from the first injection valve 35 is carriedout in an intermittent manner. Then, the increase in the amount of fuelinjected from the first injection valve 35 is carried out by at leastone of lengthening the time of fuel injection and shortening theinterval of injection. Then, the heater 36 is energized in response tothe injection of fuel from the first injection valve 35. The time ofenergization of the heater 36 may be made longer in accordance with theincreasing amount of fuel injection.

In step S105, the ECU 5 determines whether the temperature of the NOxcatalyst 4 is equal to or higher than the lower limit value of theactivation temperature. In other words, it is determined whether heatingby the temperature raising device 3 has become unnecessary. In caseswhere an affirmative determination is made in step S105, the routineadvances to step S106, whereas in cases where a negative determinationis made, the routine returns to step S104.

In step S106, the ECU 5 stops the energization of the heater 36 and theinjection of fuel from the first injection valve 35. In other words, theraising of the temperature of the exhaust gas by means of thetemperature raising device 3 is stopped. After this, NOx is reduced inthe NOx catalyst 4 by injecting the reducing agent from the secondinjection valve 37.

Here, note that the energization of the heater 36 may be stopped and thefuel injection from the first injection valve 35 may be stopped at thetime when an amount of fuel required to generate a necessary amount ofheat has been supplied.

In addition, when fuel is injected from the first injection valve 35during the time the number of engine revolutions per minute is high, theflow rate of the exhaust gas increases, so there is a fear that the fuelmay pass or sneak through the oxidation catalysts. When the fuel passesor sneaks through the oxidation catalysts, there is a fear that the fuelmay adhere to the NOx catalyst 4, thus decreasing the purificationability for NOx. Accordingly, when the number of engine revolutions perminute becomes equal to or more than a threshold value, or when theamount of exhaust gas becomes equal to or more than a threshold value,the energization of the heater 36 may be stopped and the fuel injectionfrom the first injection valve 35 may be stopped. In addition, theamount of fuel injection may also be decreased. At this time, the amountof energization or electric power to be supplied to the heater 36 andthe amount of fuel to be injected from the first injection valve 35 maybe adjusted according to the number of engine revolutions per minute orthe amount of exhaust gas. In other words, the amount energization orelectric power to be supplied to the heater 36 may be decreased, and theamount of fuel injected from the first injection valve 35 may bedecreased, in accordance with the increasing number of enginerevolutions per minute, or the increasing flow rate of exhaust gas.

Here, note that even after the temperature of the NOx catalyst 4 hasonce become equal to or higher than the lower limit value of theactivation temperature, the temperature of the NOx catalyst 4 may becomelower than the lower limit value of the activation temperature,depending on the operating state of the internal combustion engine 1. Inthis case, energization of the heater 36 and fuel injection from thefirst injection valve 35 are carried out again, thereby causing thetemperature of the NOx catalyst 4 to rise.

FIG. 4 is a flow chart showing a flow for temperature raising control onthe NOx catalyst 4 after engine starting according to this embodiment.This routine is carried out in a repeated manner at each predeterminedtime interval. Here, note that in comparison with the flow shown in FIG.3, only step S102 and step S103 are lacking, so the explanation of thisroutine is omitted.

As explained above, according to this embodiment, by the provision ofthe four oxidation catalysts in which the more downstream side they arearranged at, the larger their cross-sectional areas become, it ispossible to raise the temperature of the NOx catalyst 4 more quicklywith a small amount of fuel. In addition, the temperature of the fourthcatalyst 34 can be raised to a high temperature, so the evaporation anddispersion of the reducing agent can be facilitated. That is, thereducing agent can be supplied to the NOx catalyst 4 in a uniformmanner, while causing the temperature of the NOx catalyst 4 to rise upto the lower limit value of the activation temperature thereof in aquick manner, so the purification ability thereof for NOx can beenhanced.

Here, note that in this embodiment, the individual central axes of thefour oxidation catalyst are located on the central axis of the exhaustpassage 2, but the central axes of these catalysts may be arranged outof alignment with the central axis of the exhaust passage 2. Inaddition, the central axes of the individual oxidation catalysts may notbe on the same line. In other words, the structure may be such that theexhaust gas having passed through the inside of an upstream sideoxidation catalyst and the exhaust gas having passed through the outsidethereof flow into the following downstream side oxidation catalyst.Moreover, the guides 321, 331, 341 may be omitted.

EXPLANATION OF REFERENCE NUMERALS AND CHARACTERS

-   1 Internal combustion engine-   2 Exhaust passage-   3 Temperature raising device-   4 Occlusion reduction type NOx catalyst-   5 ECU-   11 Crank angle sensor-   12 Switch-   13 Second temperature sensor-   14 Accelerator pedal-   15 Accelerator opening sensor-   31 First catalyst-   32 Second catalyst-   33 Third catalyst-   34 Fourth catalyst-   35 First injection valve-   36 Heater-   37 Second injection valve-   38 First temperature sensor

1. An exhaust gas purification apparatus of an internal combustionengine comprising: an exhaust gas purification catalyst that is arrangedin an exhaust passage of the internal combustion engine for purifying anexhaust gas; a plurality of catalysts that are arranged at an upstreamside of said exhaust gas purification catalyst and have oxidizingability; a fuel supply device that supplies fuel to one of saidplurality of catalysts having oxidizing ability which is arranged at themost upstream side thereof; a heating device that heats said onecatalyst; wherein said plurality of catalysts having oxidizing abilityare arranged at intervals from one another in an exhaust passage inseries to the direction of flow of the exhaust gas, and the moreupstream side the catalysts are arranged, the smaller thecross-sectional areas of said catalysts formed by cutting planes whichare orthogonal to a central axis of said exhaust passage are made, andsaid cross-sectional area of at least said one catalyst is made smallerthan the cross-sectional area of said exhaust passage.
 2. The exhaustgas purification apparatus of an internal combustion engine as set forthin claim 1, wherein said exhaust gas purification catalyst Is composedof including a selective reduction type NOx catalyst which uses urea orammonia as a reducing agent; and an injection device is provided whichinjects said reducing agent toward the exhaust gas flowing out of another catalyst which is arranged at the most downstream side among saidplurality of catalysts having oxidizing ability.
 3. The exhaust gaspurification apparatus of an internal combustion engine as set forth inclaim 2, further comprising: a unit that measures or estimates thetemperature of the exhaust gas flowing out of said other catalyst,wherein when the temperature of the exhaust gas flowing out of saidother catalyst is equal to or higher than a predetermined value, thereducing agent is caused to be injected from said injection device. 4.The exhaust gas purification apparatus of an internal combustion engineas set forth in claim 1, wherein before starting of said internalcombustion engine, fuel is supplied to said one catalyst from said fuelsupply device, and said one catalyst is heated by means of said heatingdevice.
 5. The exhaust gas purification apparatus of an internalcombustion engine as set forth in claim 4, wherein after the starting ofthe internal combustion engine is commenced, the amount of fuel suppliedfrom said fuel supply device is caused to increase according to the timeelapsed.
 6. The exhaust gas purification apparatus of an internalcombustion engine as set forth in claim 4, further comprising: a unitthat measures or estimates the temperature of said exhaust gaspurification catalyst, wherein when the temperature of said exhaust gaspurification catalyst rises to a prescribed temperature, the supply offuel from said fuel supply device to said one catalyst is stopped, andthe heating of said one catalyst by said heating device is stopped. 7.The exhaust gas purification apparatus of an internal combustion engineas set forth in claim 4, wherein when the amount of the exhaust gasbecomes more than a prescribed amount during the time fuel is beingsupplied from said fuel supply device, the amount of fuel supplied fromsaid fuel supply device is restricted.